Register field name prefixes reserved by OAS 3.1.0?

[ralfhandl]

OAS 3.1.0 Section 4.9 Specification Extensions states that

Field names beginning x-oai- and x-oas- are reserved for uses defined by the OpenAPI Initiative.

Should we add these to the Namespace Registry?

[miqui]

@ralfhandl I don't see a problem with this. At least it raises visibility on the OAI/OAS side.